The present invention relates to a camera which automatically operates by controlling an electric motor and, more particularly, to a camera driving stop apparatus which performs precision control to stop, at a desired target position, a driving mechanism for performing zooming and focusing of a photographing lens, film feed, switching of the driving force transmission path, and the like.
In conventional camera zooming or focusing driving control, when a driven unit comes close to a target position, its driving speed is reduced in order to stop it at the target position as accurately as possible, as described in Jpn. UM Appln. KOKOKU Publication No. 7-49455 (U.S. Pat. No. 4,777,504).
In the above control, if the electric motor comes to stop immediately before the driven unit reaches the target position, the electric motor is restarted so as to allow the driven unit to reach the target position.
In this prior art, the driving speed is simply reduced until the driven unit reaches the predetermined target position, or the electric motor is restarted. For this reason, the driven unit is influenced by the ambient temperature, the posture of a driving system, aging deterioration, the mechanical time constant of the driving system, and the like, resulting in variations in the final stop position of the driven unit.
The reason why the stop position of the driven unit varies will be described with reference to FIGS. 12A and 12B.
FIGS. 12A and 12B are timing charts, respectively, showing the waveform of the output pulse from a pulse generation means which is attached to the driven unit such as a focusing mechanism for a photographing lens, and operates in response to movement of this driven unit, and the ON/OFF state of the motor which drives the driven unit at this time.
FIG. 12A shows that the driven means properly overruns and stops at a target position (n+N) upon turning off the ON motor when the count value, 1, . . . , n-1, n, n+1, . . . , n+(N-1), n+N, of output pulses from the pulse generation means reaches a predetermined pulse count (n) before the target position.
That is, the stop position coincides with the target position in this case.
If the moving speed of the driving system is known, the overrunning pulse amount (N) can be predicted, so that control in this form is possible.
In fact, however, the driven unit may stop before the target position due to changes in ambient temperature, posture differences of the driving system, aging deterioration, or the like, resulting in variations the final stop position.
For this reason, as shown in FIG. 12B, if the count value does not reach the predetermined pulse count (n+N) a predetermined time a after the pulse edge at a point n+(N-1) upon turning off the motor at the predetermined pulse count (n), control is performed to restart the motor in order to drive the driven means to the target position, and to stop the motor after it is detected that the driven unit has indeed reached the target position (n+N).
Under this control, however, the response is delayed owing to the mechanical time constant of the driving system, and the driven unit undesirably stops at a position n+(N+x) past the target position.
That is, even if the motor is energized immediately before the target position, and turned off when the driven unit reaches the target position, since excess energy has been supplied to the driving mechanism including the motor due to the mechanical time constant of the driving system before the driven unit reaches the target position, the driven unit overruns by a distance corresponding to the excess energy, and stops.